1. Technical Field
The present invention relates to an apparatus for diagnosing a fuel pressure sensor characteristic fault which diagnoses the presence of a fault in a fuel pressure sensor that is provided in a fuel system to detect the pressure of fuel.
2. Related Art
In direct injection engines, an injector (fuel injector) is provided for each cylinder to directly inject fuel into the cylinder, fuel stored in a fuel tank is pressurized by a low-pressure pump and then further pressurized by a high-pressure fuel pump (high-pressure fuel generator) before being introduced into a high-pressure fuel gallery, and the fuel is directly injected into the cylinder from the injector that communicates with this high-pressure fuel gallery.
The fuel pressure supplied to the high-pressure fuel gallery is controlled by a controller (ECU). In the controller, first, a target fuel pressure is set in accordance with the running condition of the engine, and feedback control is performed so that the actual fuel pressure detected by a fuel pressure sensor converges to the target fuel pressure. Then, fuel is injected from the injector into the cylinder for the duration of a fuel injection time corresponding to a target fuel injection quantity, which is set in accordance with the fuel pressure detected by the fuel pressure sensor and the running condition of the engine.
To provide optimal control of the fuel pressure supplied to the high-pressure fuel gallery, and fuel injection quantity, it is necessary for the output characteristics of the fuel pressure sensor to fall within a predetermined tolerance range. Further, a break or ground fault in the lead wire of the fuel pressure sensor makes the fuel pressure sensor unable to give an accurate fuel pressure reading.
For example, Japanese Patent No. 3966130 discloses the following technique. According to the technique, when a preset soak time (the time from engine stop to turn-on of a key switch) elapses, it is determined that the fuel pressure supplied to the high-pressure fuel gallery (common rail) has dropped to a pressure equivalent to the atmospheric pressure, thus enabling fault determination (characteristic diagnosis) for the fuel pressure sensor (common rail pressure sensor). Then, it is determined whether the fuel pressure stored in the high-pressure fuel gallery falls outside a predetermined range, and if this fuel pressure is outside the predetermined range, it is determined that a fault in the characteristics of the fuel pressure sensor (to be also referred to as “characteristic fault” hereinafter) is present on the low output side.
According to the technique disclosed in Japanese Patent No. 3966130 mentioned above, when determining a characteristic fault in the fuel pressure sensor, if the soak time is longer than a preset time, it is estimated that the fuel pressure supplied to the high-pressure fuel gallery (common rail) has dropped to a pressure equivalent to the atmospheric pressure.
However, the time required for the fuel pressure supplied to the high-pressure fuel gallery to drop to a pressure equivalent to the atmospheric pressure varies greatly with the operating condition immediately before engine stop. Therefore, if the preset time is short, this does allow the fuel pressure in the high-pressure fuel gallery to sufficiently drop to a pressure equivalent to the atmospheric pressure, making it impossible to perform a characteristic fault diagnosis of the fuel pressure sensor. Accordingly, it is necessary to set the preset time mentioned above to such a length of time that allows the fuel pressure supplied to the high-pressure fuel gallery to sufficiently drop to a pressure equivalent to the atmospheric pressure. This means that the characteristic fault diagnosis may not be performed unless the soak time is relatively long, reducing the opportunities for diagnosis.
It is preferable to start a diagnosis of a characteristic fault in the fuel pressure sensor is preferably as early as possible after engine stop. This reduces the influence of disturbance, allowing high accuracy diagnosis.
The technique disclosed in Japanese Patent No. 3966130 above also proposes a technique in which, instead of using a set time that is compared with the soak time, the fuel pressure in the high-pressure fuel gallery is estimated to have dropped to a pressure equivalent to the atmospheric pressure if one of the coolant temperature, the intake temperature, the fuel temperature, and the engine oil temperature after engine stop has dropped by a predetermined value or more. However, detecting the coolant temperature or the like at turn-on of the key switch, and estimating if the fuel pressure has dropped to a pressure equivalent to the atmospheric pressure on the basis of the detected temperature means that a complex condition needs to be satisfied in order to execute the diagnosis (to be also referred to as “diagnosis execution condition” hereinafter).
Furthermore, with the technique disclosed in Japanese Patent No. 3966130 above, it is not possible to discriminate whether the faulty condition of the fuel pressure sensor is due to a characteristic fault or a ground fault.